Getting the Right Twist: Influence of Donor-Acceptor Dihedral Angle on Exciton Kinetics and Singlet-Triplet Gap in Deep Blue Thermally Activated Delayed Fluorescence Emitter

Here, a novel deep blue emitter SBABz4 for use in organic light-emitting diodes (OLED) is investigated. The molecular design of the emitter enables thermally activated delayed fluorescence (TADF), which we examine by temperature-dependent time-resolved electroluminescence (trEL) and photoluminescence (trPL). We show that the dihedral angle between donor and acceptor strongly affects the oscillator strength of the charge transfer state alongside the singlet-triplet gap. The angular dependence of the singlet-triplet gap is calculated by time-dependent density functional theory (TD-DFT). A gap of 15 meV is calculated for the relaxed groundstate configuration of SBABz4 with a dihedral angle between the donor and acceptor moieties of 86 degrees. Surprisingly, an experimentally obtained energy gap of 72 +/- 5 meV can only be explained by torsion angles in the range of 70-75 degrees. Molecular dynamics (MD) simulations showed that SBABz4 evaporated at high temperature acquires a distribution of torsion angles, which immediately leads to the experimentally obtained energy gap. Moreover, the emitter orientation anisotropy in a host shows an 80% ratio of horizontally oriented dipoles, which is highly desirable for efficient light outcoupling. Understanding intramolecular donor-acceptor geometry in evaporated films is crucial for OLED applications, because it affects oscillator strength and TADF efficiency.

A Theoretical Rationalisation of the Emission Properties of Prototypical Cu(I)-Phenanthroline Complexes

Gloria Capano, Thomas James Penfold, Ursula Röthlisberger, Ivano Tavernelli

The excited state properties of transition metal complexes have become a central focus of research owing to a wide range of possible applications that seek to exploit their luminescence properties. Herein, we use density functional theory (DFT), time-dependent DFT (TDDFT), classical and quantum mechanics/molecular mechanics (QM/MM) molecular dynamics (MD) simulations to provide a full understanding on the role of the geometric and electronic structure, spin orbit coupling, singlet triplet gap and the solvent environment on the emission properties of nine prototypical copper(I)-phenanthroline complexes. Our calculations reveal clear trends in the electronic properties that are strongly correlated to the luminescence properties, allowing us to rationalize the role of specific structural modifications. The MD simulations show, in agreement with recent experimental observations, that the lifetime shortening of the excited triplet state in donor solvents (acetonitrile) is not due to the formation of an exciplex. Instead, the solute solvent interaction is transient and arises from solvent structures that are similar to the ones already present in the ground state. These results based on a subset of the prototypical mononuclear Cu(I) complexes shed general insight into these complexes that may be exploited for development of mononuclear Cu(I) complexes for applications as, for example, emitters in third generation OLEDs.

2015

Structure and dynamics of liquid water from ab initio molecular dynamics - Comparison of BLYP, PBE and revPBE density functionals with and without van der Waals corrections

I-Chun Lin, Ursula Röthlisberger, Ivano Tavernelli

We investigate the accuracy provided by different treatments of the exchange and correlation effects, in particular the London dispersion forces, on the properties of liquid water using ab initio molecular dynamics simulations with density functional theory. The lack of London dispersion forces in generalized gradient approximations (GGAs) is remedied by means of dispersion-corrected atom-centered potentials (DCACPs) or damped atom-pairwise dispersion corrections of the C6R–6 form. We compare results from simulations using GGA density functionals (BLYP, PBE, and revPBE) with data from their van der Waals (vdW) corrected counterparts. As pointed out previously, all vdW-corrected BLYP simulations give rise to highly mobile water whose softened structure is closer to experimental data than the one predicted by the bare BLYP functional. Including vdW interactions in the PBE functional, on the other hand, has little influence on both structural and dynamical properties of water. Augmenting the revPBE functional with either damped atom-pairwise dispersion corrections or DCACP evokes opposite behaviors. The former further softens the already under-structured revPBE water, whereas the latter makes it more glassy. These results demonstrate the delicacy needed in describing weak interactions in molecular liquids.

Amer Chemical Soc2012

Structure determination of an amorphous drug through large-scale NMR predictions

Martins Balodis, Pierrick Berruyer, Manuel Cordova, David Lyndon Emsley, Albert Hofstetter, Federico Maria Paruzzo, Bruno Simões de Almeida

Knowledge of the structure of amorphous solids can direct, for example, the optimization of pharmaceutical formulations, but atomic-level structure determination in amorphous molecular solids has so far not been possible. Solid-state nuclear magnetic resonance (NMR) is among the most popular methods to characterize amorphous materials, and molecular dynamics (MD) simulations can help describe the structure of disordered materials. However, directly relating MD to NMR experiments in molecular solids has been out of reach until now because of the large size of these simulations. Here, using a machine learning model of chemical shifts, we determine the atomic-level structure of the hydrated amorphous drug AZD5718 by combining dynamic nuclear polarization-enhanced solid-state NMR experiments with predicted chemical shifts for MD simulations of large systems. From these amorphous structures we then identify H-bonding motifs and relate them to local intermolecular complex formation energies. Determining the structure of amorphous solids is important for optimization of pharmaceutical formulations, but direct relation of molecular dynamics (MD) simulations and NMR to achieve this is challenging. Here, the authors use a machine learning model of chemical shifts to solve the atomic-level structure of the hydrated amorphous drug AZD5718 by combining dynamic nuclear polarization-enhanced solid-state NMR with predicted shifts for MD simulations of large systems.

NATURE RESEARCH2021